Sulphate cooking of cellulose pulp produces malodorous organic sulphur compounds, such as methyl mercaptan, dimethyl sulphide and dimethyl disulphide, as the sulphide and hydrogen sulphide ions react with lignin and the methoxyl groups of lignin fragment. Sulphate cooking also produces a large amount of methanol mainly in alkaline hydrolysis of lignin. Large amounts of vapors containing methanol and sulphur compounds are released, e.g., in the evaporation of black liquor, in which the above-mentioned compounds are distilled and condensed into the condensates of a multi-effect evaporation plant. Methanol has a high COD (Chemical Oxygen Demand), and therefore methanol is removed to lower the COD-level.
It has been very important in order to comply with environmental regulations in the United States to deal with the "Cluster Rules" established by the Environmental Protection Agency.
The Cluster Rules basically say that a cellulose pulp mill (e.g. kraft mill) has to collect streams of condensate resulting from the pulping operation containing at least 65% of the HAP's (by EPA defined as MeOH which is a surrogate for real HAP's which might be more than 50 different kinds of components) and to treat these condensates so that 92% of this HAP (MeOH) is removed and destroyed by means of thermal oxidation.
The mill may also elect to show that condensates containing at least 7.2 lb HAP/ODTP for unbleached pulp or 11.1 lb HAP/ODTP for bleached pulp have been collected into a stream(s) which can be treated to remove 6.6 lb HAP/ODTP and 10.2 lb HAP/ODTP, respectively, or to achieve a maximum HAP (MeOH) outlet concentration of 210 ppmv and 330 ppmv, respectively.
In part due to the unique design of a plate type falling film evaporator, such as available from Ahlstrom Recovery, Inc., of Alpharetta, Ga., in combination with low speed steam compression technology and integral steam stripping, it is possible, according to the invention, to provide a one-step solution to comply with the "Cluster Rules", although the invention can also be practiced with other conventional, e.g. tubular, heat transfer surfaces.
Essentially separated from the existing operation, the invention comprises or consists of two sets (sections) of heating elements, separated on the motive steam side with common vapor release. Weak liquor from the pulp mill is evaporated for the first time in a single stage VCE unit, guaranteeing the release of at least 65% of the HAP (MeOH) in the spent pulping liquor.
According to the invention the vapor containing MeOH is compressed in a first compressor and then flows through the main section of the heating elements of the evaporator. A major part (e.g. 80%) of the vapor is condensed in a countercurrent fashion and the rest (e.g. 20%) of the vapor is vented. The vented vapor is compressed in a secondary compressor and fed a stripper reboiler where it is condensed. By this method, at least 80% (e.g. 90%) of the 65% MeOH will end up in the stripper reboiler condensate.
The second section of the heating elements is fed with overhead vapor from the steam stripper which is fed with reboiled stripped condensate produced in a reboiler/evaporator similar to the one previously described. To overcome the additional .DELTA.p, the second compressor is used to compress the vapor from the main evaporator/compressor.
The stripper will receive the condensates condensed in the above-described reboiler as well as the condensates from the condensing of digester blow-off vapor, turpentine underflow, and the NCG collection system. By collecting and treating all of these sources, a pulp mill can satisfy the required collection and treatment procedures of the Cluster Rules.
According to the invention a method of treating cellulose pulp mill condensates having a MeOH content is provided, using an evaporator and a steam stripper having a reboiler. The method typically comprises: (a) Collecting a first feed liquor stream having at least about 50% of the pulp mill MeOH (methanol); (b) Feeding the first feed liquor stream to an evaporator having at least two heating element sections separated on the motive steam side; (c) Evaporating the liquor in the evaporator to produce a vapor containing at least about 40% of the at least 50% of the pulp mill MeOH; (d) Compressing the vapor from step (c) to increase the vapor pressure thereof; (e) Using the vapor from step (d) as condensing heating media in step (c) for the evaporation in a first heating element section of the evaporator and venting a portion of the vapor through the first heating element section; (f) Compressing the vented vapor from the heating elements of the evaporator from step (e) to increase the vapor pressure, so as to allow it to be used as heating media in the reboiler; (g) Condensing the compressed vapor from step (f) in the reboiler while evaporating already stripped condensate fed to the reboiler so that vapor formed in the reboiler flows to the stripper; (h) Feeding the condensate from the reboiler and condensate from the evaporator to the stripper column; (i) Causing vapor from step (g) to enter the stripper column and to flow upwards countercurrent to the condensate from step (h), producing an MeOH rich vapor leaving the stripper column; (j) Causing the MeOH rich vapor to enter a second heating element section of the evaporator and condense while evaporating the liquor in step (c) producing a condensate entering the stripper in step (h); and (k)Venting a portion of the MeOH rich vapor entering the heating element in step (j). The method may comprise a further step (l) of leading the MeOH-rich vapor from step (k) to a thermal destruct unit or to a methanol liquefaction plant.
According to one exemplary embodiment of the invention in step (e) the vapor containing MeOH is condensed in a countercurrent fashion in a two-stage heating element section to produce a small stream of condensate with high HAP (MeOH) content (e.g. &gt;90% of the HAP's) to be fed to the stripper and a large stream (containing e.g. &lt;8% of the HAP's ). The balance of HAP's will be contained in a vent stream from the evaporator.
It is the primary object of the present invention to provide a method that allows a pulp mill to satisfy the required collection and treatment procedures of the Cluster Rules. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims